To further characterize beta 2-microglobulin (beta 2m) on the surface of human lymphocytes and to study the relationship of this molecule to other membrane bound antigens, especially HL-A and other histocompatibility antigens, including both serologically defined and lymphocyte defined antigens. To explore the possibility that either the beta 2m-HL-A complex or beta 2m and the Ir gene product functions as the antigen receptor on T cells and to investigate the possible role of beta 2m in other manifestations of T cell function, i. e., helper function, killer cell activity, delayed hypersensitivity. To investigate the extent of genetic polymorphism among beta 2m by comparative biochemical procedures such as amino acid composition analysis, peptide mapping to determine the existence of allotypes and/or the existence of V regions similar to that in the immunoglobulins. To define beta 2m in the mouse system and to study the relationship of membrane bound beta 2m to H2 antigens and to other products of genes closely linked to the H2 locus. As in the human system, the possibiliy that the beta 2m-H2 complex or beta 2m-Ir gene product complex serves as the antigen receptor on T cells and the role of beta 2m in other manifestations of T cell functions will be explored. To investigate the clinical value of a quantitative assay for serum beta 2m levels in situations in which graft rejection phenomena are occurring, as a sensitive indicator, at a subclinical level, of the onset of graft rejection due to immunological attack on histocompatibility antigens.